The Bucks, who were are currently rock bottom of the National League North, were in need of a boost in the FA Trophy but it got off to a bad start in Hertfordshire.
Just 17 minutes in and a free kick from the right was met by James Kaloczi and he powered a header past Luke Pilling to hand the hosts the lead.
Brendon Daniels went close to an equsliser just before the half hour mark before Kobe Jae Chong lashed home from 25 yards out to level things up for the Bucks.
Both sides had chances in the second period - with both Daniels and Jason Oswell going close, but the Bucks were dealt another blow as the forward was taken off late on with a shoulder injury.
Then just as it look as though the tie was heading for a draw - captain Brown rose in the box to meet a set piece and power a header home to secure the Bucks' passage to the next round.